In radio and telecommunications a dipole antenna is the simplest and most widely used class of antenna. In its simplest form, it consists of two identical conductive elements, such as metal wires or rods, which are usually bilaterally symmetrical. The driving current from the transmitter is applied, or for receiving antennas the output signal to the receiver is taken, between the two halves of the antenna. Each side of the feed line to the transmitter or receiver is connected to one of the conductors.
The most common form of dipole is two straight rods or wires oriented end to end on the same axis, with the feed line connected to the two adjacent ends. Dipoles are resonant antennas, meaning that the elements serve as resonators, with standing waves of radio current flowing back and forth between their ends. The length of the dipole elements is determined by the wavelength of the radio waves used. The most common form is the half-wave dipole, in which each of the two rod elements is approximately ¼ wavelength long, and the whole antenna is a half-wavelength long. The radiation pattern of a vertical dipole is omnidirectional; it radiates equal power in all azimuthal directions perpendicular to the axis of the antenna. For a half-wave dipole the radiation is maximum, 2.15 dBi perpendicular to the antenna axis, falling monotonically with elevation angle to zero on the axis, off the ends of the antenna.
Several different variations of the dipole are also used, such as the folded dipole, short dipole, cage dipole, bow-tie, and batwing antenna. Dipoles may be used as standalone antennas themselves, but they are also employed as feed antennas (driven elements) in many more complex antenna types, such as the Yagi antenna, parabolic antenna, reflective array, turnstile antenna, log periodic antenna, and phased array.
FIG. 1 is a schematic representation of a center fed, multiband dipole antenna. In FIG. 1, a high band dipole element 10 and a low band dipole element 12 are connected to a transmitter and/or receiver at a center feed point 11. The antenna in FIG. 1 is a parallel dipole. In the parallel design, several dipoles are joined together in the center and fed with the same cable. The dipole that radiates the RF is the one that presents an impedance that most closely matches the cable (50 ohms). That matching impedance changes according to the frequency of the signal. One dipole offer a 50-ohm match on, e.g. a low band, while another provides the best match on, e.g. a high band.
FIG. 2 is a schematic representation of an end and center fed, multiband dipole antenna. In FIG. 2, a high band dipole element 20 and a low band dipole element 22 are respectively connected to a transmitter and/or receiver at one of an end feed point 23 and a center feed point 21.
A dipole is a symmetrical antenna because it is composed of two symmetrical ungrounded elements. Therefore, it works best when fed by a balanced transmission line because in that case the symmetry matches and therefore the power transfer is extremal. When a dipole with an unbalanced feed line such as coaxial cable is used for transmitting, the shield side of the cable, in addition to the antenna, radiates. This can induce radio frequency (RF) currents into other electronic equipment near the radiating feed line, causing RF interference. Furthermore, the antenna is not as efficient as it could be because it is radiating closer to the ground and its radiation pattern may be asymmetrically distorted. At higher frequencies, where the length of the dipole becomes significantly shorter than the diameter of the feeder cable, this becomes a more significant problem. To prevent this, dipoles fed by coaxial cables have a balun between the cable and the antenna, to convert the unbalanced signal provided by the coax to a balanced symmetrical signal for the antenna. FIG. 3 is a schematic representation of a balanced multiband dipole antenna that is connected at a center feed 30 point to a transmitter and/or receiver via a balun 31 to a coax feed 32.
FIG. 4 is a schematic representation of a balanced end fed, multiband trap dipole antenna. An antenna trap can be described as being an application of a resonant circuit. Such a trap, using a parallel resonant tuned circuit, offers very high impedance at or near resonance and acts as an insulator to effectively cut off the element to a length resonant to the band being used. In FIG. 4, an end feed coax 40 has a coax center conductor contact point 43. A low band trap 41 and a high band trap 42 allow the antenna to resonate properly at both high band and low band frequencies.